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Applied Physics B

Axial and conical parametric emissions from potassium atoms under two-photon fs excitation

Dionysios Pentaris, Thomas Efthimiopoulos, Nikolaos Merlemis, Virgilijus Vaičaitis

Published Oct 31, 2009• 6 citations
Publisher
DOI
10.1007/s00340-009-3801-8

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Journal of Physics B Atomic Molecular and Optical Physics•Nov 28, 1996•9 citations
The interaction of a laser with potassium atoms is investigated by monitoring the radiation of the 5P-4S transition under the two-photon 4S(1/2)-6S(1/2) excitation. For low number density and laser intensity the observed radiation is parametric axial or conical, depending on the laser detuning from the two-photon resonance, the number density and the laser intensity. With increasing laser intensities and number densities, an additional delayed component is observed corresponding to a two-step four-wave mixing with the participation of internally generated photons. A two-photon quantum interference effect is observed for the total (axial and conical) parametric four-wave mixing emission. A deviation from the proposed model under certain experimental parameters is attributed to a two-step four-wave mixing and the phase mismatch of the parametric processes.
The combined excitation of high density potassium (K) vapour by 100 fs pump-coupling pulses is experimentally studied. The intense pump pulse excites the two-photon 4S1/2↔6S1/2 transition and internally generated emissions are initiated along the atomic paths: 6S1/2↔5P3/2,1/2↔4S1/2 (path-1) and, 6S1/2↔4P3/2,1/2↔4S1/2 (path-2). The temporally delayed coupling pulse coherently drives the 6S1/2↔4P3/2,1/2 transitions, in a Λ-type excitation scheme. The competing axial and conical emission components of the well-resolved 4P3/2,1/2↔4S1/2 transitions (D2 and D1 lines of K) are substantially enhanced and controlled, for appropriate detunings and pump-coupling temporal delays. The coherence relaxation time (CRT) of the two-photon excited 6S1/2 state is determined by exploiting the temporal delay in the pulse sequence. The effect of the pulse delay and the fs pulse bandwidth on the system dynamics is discussed as well as the role of dephasing collisions between K and buffer gas atoms. The proposed scheme can be employed in radiative multi-level systems, for the direct estimation of coherence relaxation rates of various states.
It is shown that by proper use of the excitation spectra, the emission spectra of the transition and the far-field patterns it is possible to distinguish between competing processes involved in the two-photon excitation to the 6S state of potassium, namely: (a) resonant fluorescence emission associated with the cascading spontaneous emission (ASE) and (b) the four-wave mixing parametric process. The effect of the buffer gas on the intensities of the spectra of the radiations emitted is discussed. Conical emission and spatial instabilities of the excitation radiation and the emission were also observed for high excitation and high vapour pressure of potassium.